We spoke to the people at Mitsubishi Electric Home Appliance Co., Ltd. who worked on the design and development of its advanced IH jar rice cooker, Hon Sumigama KAMADO®.
How to Recreate the Texture of Oven-Cooked Rice
To find out just why rice cooked in a kamado oven tastes so delicious, we conducted repeated experiments and uncovered the reasons scientifically. In those experiments, we focused our attention on the texture of the cooked rice, namely its firmness and stickiness. We also looked at its moistness. Usually, rice with a high water content is soft, and rice with a low water content is firm. However, oven-cooked rice is moister than rice cooked in a rice cooker, yet it still maintains its firmness. This can be achieved in a kamado oven because the rice is cooked at a high temperature for a short time. Therefore we concluded that to get close to the texture of oven-cooked rice with a rice cooker, very high heat would be essential.
Cross-sectional model of IH jar rice cooker
The inner pot is heated by seven heaters
How to Achieve High Heat
The IH jar rice cooker has an induction heating coil in its base, which creates a magnetic field and heats up the inner pot of the rice cooker. The inner pots of rice cookers are usually made of stainless steel or other metal materials, but for the last ten years, Mitsubishi Electric has been using carbon materials for the entire inner pot. Carbon materials are characterized by the depth of permeation of the magnetic flux, that is, how deep into the wall thickness heat is generated. With carbon materials, heat is generated to a maximum depth of 10 mm (while regular stainless steel bowls heat up only to around 0.25 mm). This means that the full thickness of the inner pot heats up. Moreover, carbon materials have a higher thermal conductivity than metal, so the heat generated is transmitted more readily. Heaters have also been installed on the sides and top of the rice cooker, thus achieving the required high temperature. Another innovation was to incorporate the traditional hagama shape of a kamado pot for the inner pot. This shape has a supporting brim in the center with sections above and below, so the rice can cook without boiling over, even at very high temperatures.
Coil base made with PPS (top)
Coil base made with PPS (underside)
Adoption of DURAFIDE® PPS
To achieve the higher heat required, it was necessary to review the materials used in the rice cooker from the perspective of heat resistance, specifically the coil base, which had until then been made with polyethylene terephthalate (PET). This material needed to be replaced with an engineering plastic that offered higher heat resistance. After much consideration, the material we chose was Polyplastics' DURAFIDE® PPS. Because the coil base is considered to be an electric insulator, our in-house standards required that we obtain CMJ registration* from Japan Electrical Safety & Environment Technology Laboratories (JET). Verification of the upper limits of usage temperatures of the insulating materials revealed that PPS has an upper range of 180-200℃, compared to PET's 150-160℃. In order to achieve the desired high heat, the coil base would have to reach a temperature of close to 170℃, and we learned that parts made with PPS would be able to withstand that temperature.
* CMJ registration: a system in which parts and materials are registered in advance for the economical and efficient approval of electrical products.
Completion of the PPS Coil Base
The grade of PPS material we adopted is reinforced with a glass-fiber inorganic filler. Besides its ability to withstand high temperatures, another decisive factor in the adoption of this material was its favorable external appearance. This was because part of the coil base would be visible when the inner pot was removed from the rice cooker. In general, glass fiber-reinforced materials present molding difficulties due to the orientation of the glass fibers and other factors. Also, due to the shape and design of the coil base, we faced limitations on the position of the gate as well*1. These limitations led to unstable flowability, and in the first molding attempts, the material did not flow well and the external appearance was unacceptable. The temperature of the mold also needed to be higher for PPS than for PET, but we also struggled with this because we did not have the necessary equipment in place. In addition to bringing in equipment such as pressurized mold temperature regulators, we also asked Polyplastics' TSC*2 engineers to observe the molding trials. They gave us some proposals for the optimal processing parameters and suggestions for modifying the molds. They also proposed a grade of material that, although slightly more expensive, offered superior flowability and external appearance. With the adoption of that grade, we were finally able to satisfy both the external appearance and material property requirements, and development of the part was completed.
*1 The hole where the plastic flows into the mold cavity
*2 Technical Solution Center in Fuji, Shizuoka
The Hon Sumigama KAMADO® could be described as the ultimate in rice cookers available today, but Mitsubishi Electric Home Appliance will continue its pursuit of even better taste and the development of ever better rice cookers. Our aim at Polyplastics is to enrich consumers' daily lives by assisting our customers with their product development.
* "Hon Sumigama KAMADO®" is a registered trademark of Mitsubishi Electric Home Appliance Co., Ltd.
Best Partner for Development
Unit 2. Japan Sales Headquarters
Global Sales and Marketing Division
We strive to understand our customers' development timeframes and offer the best technical proposals to meet their requirements in a timely manner. Speedy support is of particular importance for home appliances, which have a relatively short development cycle. Our success on this occasion was thanks in part to the cooperation we received from a local trading company. It is essential to visit electrical superstores frequently to monitor the latest technical and product trends. We will continue our efforts to support our customers in their new challenges and to be the best partner possible to help them achieve their goals.
Mitsubishi Electric Home Appliance Co., Ltd.
Products: vacuum cleaners, microwaves/broilers (microwaves/convection ovens), rice cookers, dishwasher-dryers (built-in type), IH cooking heaters, dish dryers, toaster ovens, futon dryers, dehumidifiers, air purifiers, rapid de-odorizers.